Temperature as a function of radius for the four different models produced with Transphere. Black solid line: with a central source of 4   L_⊙ and an interstellar radiation field with χ_ISRF = 750. Dotted line: with a central source of 4   L_⊙ and a normal interstellar radiation field (χ_ISRF = 1). Dashed line: without a central source, but with an interstellar radiation field with χ_ISRF = 750. Grey solid line: as the black solid line, but with an ambient density of 10⁵ cm^-3.